Demonstration of cytoplasmic actomyosin fibrils by the freeze-etching technique.

Cytoplasmic actomyosin fibrils of Physarum polycephalum were demonstrated by the freeze-etching technique. Within the fibrils the electron micrographs reveal 40-80 A thick filaments presumably representing F-actin. Thicker filaments (myosin) could not be detected. To avoid freezing artefacts, a pretreatment of the living material with 50% sucrose was necessary. The influence of freeze-protecting agents upon the fine structure of cytoplasmic actomyosins and the resulting difficulties in the investigation of constituents of the groundplasm with the freeze-etching technique are discussed.